1. Field of the Invention
The present invention relates to projection objectives of microlithographic projection exposure apparatus, such as those used for the production of integrated circuits and other microstructured components.
2. Description of the Prior Art
For the production of large-scale integrated electrical circuits and other microstructured components, a plurality of structured layers is applied on a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of structures, which is located on a mask, is thus illuminated by an illumination system and imaged onto the photoresist by a projection objective. Since the imaging scale is generally less than 1, such projection objectives are often also referred to as reduction objectives.
After the photoresist has been developed, the wafer is subjected to an etching or deposition process so that the top layer becomes structured according to the pattern on the mask. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied on the wafer.
The size of the structures which can be defined depends primarily on the resolution of the projection objective being used. Since the resolution of the projection objective increases as the wavelengths of the projection light become shorter, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths used at present are in the deep ultraviolet (DUV) spectral range, namely 193 nm and 157 nm.
Another way of increasing the resolution is based on the idea of introducing an immersion medium with a high refractive index into the intermediate space which remains between a last lens on the image side of the projection objective and the photoresist, or another photosensitive layer to be exposed. Projection objectives which are specially designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures (NA) of more than 1, for example 1.3 or 1.4, on the image side. Moreover, immersion not only allows high numerical apertures and therefore an improved resolution, but also has a favorable effect on the depth of focus. The requirements for exact positioning of the wafer in the image plane of the projection objective are commensurately less stringent when the depth of focus is greater.
The immersion medium is generally a liquid. Solid immersion media have nevertheless been considered as well (solid immersion). The solid immersion medium does not then enter into direct contact with the photosensitive layer, but remains separated from it by a very narrow gap whose thickness is merely a fraction of the wavelength of the projection light being used. For the sake of simplicity, it will be assumed below that the immersion medium is an immersion liquid. Nevertheless, corresponding considerations also apply to solid immersion media.
The use of immersion liquids whose refractive index is more than the refractive index of the material of the last lens on the image side has now become established. In this way, it is possible to achieve a numerical aperture on the image side which is more than the refractive index of this lens material. If the lens material is quartz glass, for example, which has a refractive index nSiO2 of approximately 1.56 at a wavelength of 193 nm, then it is possible to achieve numerical apertures of 1.6 or more on the image side. This is sometimes also referred to as high index immersion lithography (HIIL).
In HIIL immersion objectives, the surface on the image side of the last lens must be concavely curved. Only then is it possible to couple the projection light into the higher-index immersion medium without sizeable light losses occurring because of total reflection at the interface between the last lens on the image side and the immersion liquid. The effect of the concave curvature of the last surface on the image side is that the immersion liquid forms a liquid lens with a positive refracting power between the wafer and the projection objective.
Examples of the structure of immersion objectives with such a concavely curved last surface on the image side can be found in WO 2005/081067, WO 2005/059617 and WO 2005/059654.
When using such HIIL immersion objectives, however, it has been found that it is difficult to ensure a consistently high imaging quality during the projection operation. Similar problems moreover occur in projection objectives with liquid lenses which lie inside a projection objective.